The cause of Sigwart's error is his misconception of " all."
So Sigwart's syllogism would be the following non-sequitur:- If anything is a professor, it is good.
These very different relations of premises are obliterated by Sigwart's false reduction of categorical universals to hypotheticals.
But even Sigwart's errors are outdone by Lotze, who not only reduces " Every NI is P " so " If S is M, S is P," but proceeds to reduce this hypothetical to the disjunctive, " If S is NI, S is P L or P 2 or 1 33, " and finds fault with the Aristotelian syllogism because it contents itself with inferring " S is P " without showing what P. Now there are occasions when we want to reason in this disjunctive manner, to consider whether S is I n or P 2 or P 3, and to conclude that " S is a particular P "; but ordinarily all we want to know is that " S is P "; e.g.
But Kant's criticism and Sigwart's corollary only derive plausibility from a false definition of truth.
In indicating specifically, too, the case of conclusion from a copulative major premise with a disjunctive minor, Herbart seems to have suggested the cue for Sigwart's exposition of Bacon's method of exclusions.
Sigwart's aim was " to reconstruct logic from the point of view of methodology."
2 It is characteristic of Sigwart's point of view that he acknowledges obligation to Mill as well as to Ueberweg.